The present invention relates to a fault diagnosis method for an input speed sensor of an automatic transmission for a vehicle, and more particularly, to a method that makes it possible to diagnose a fault of the input speed sensor of a vehicle without a vehicle speed sensor when a shift lever is positioned not only in forward running ranges, but also in non-running ranges, and also when the vehicle is operating at a low engine speed.
Generally, an ECU (Electronic Control Unit) generates a fault diagnosis code for indicating faults in an engine control process and units thereof on the basis of a vehicle state that is detected by several sensors, such as a vehicle speed sensor and an engine speed sensor.
FIG. 1 schematically shows signal processing executed by the ECU.
As shown in FIG. 1, a sensing means for detecting data on a vehicle state comprises a vehicle speed sensor 11 detecting a vehicle speed, an inhibitor switch 12 detecting a shift lever position, an oil temperature sensor 13 detecting an automatic transmission fluid temperature, a brake switch 14 detecting a brake pedal application, an engine speed sensor 15 detecting an engine speed, an input shaft speed sensor 17 detecting an input shaft speed of a transmission, and an output shaft speed sensor 18 detecting an output shaft speed of the transmission.
An ECU 20 determines driving conditions or a vehicle state by a predetermined control logic or diagnosis logic on the basis of data detected by the above sensing means, and generates a diagnosis code for the units that are malfunctioning.
If there are malfunctioning units, the ECU 20 outputs an error code that is allotted to each malfunctioning unit and informs a driver of the fault by lighting an MIL (Malfunction Indication Lamp) 30.
The ECU 20 determines whether the input shaft speed sensor 17 is malfunctioning by diagnosing an output signal of the input shaft speed sensor when a forward running range signal is input from the inhibitor switch 12 and a vehicle speed detected by the vehicle speed sensor 11 is higher than a predetermined speed. If the signal of the input shaft speed sensor 17 is not present, the ECU determines that the input shaft speed sensor is malfunctioning. The forward running range is D, 3, 2, L, sports mode, or the like.
Also, the ECU 20 determines whether the input shaft speed sensor 17 is malfunctioning by diagnosing a signal of the input shaft speed sensor when the forward running range signal is input from the inhibitor switch 12, and an engine speed is higher than an engine stall RPM. If the signal of the input shaft speed sensor is not present, the ECU determines that the input shaft speed sensor is malfunctioning.
However, in the aforementioned systems, the ECU determines whether the input shaft speed sensor is malfunctioning on the basis of the vehicle speed sensor signal. Therefore, if the vehicle speed sensor is removed, the fault diagnosis method for the input shaft speed sensor cannot be realized. Recently, there has been a tendency to remove the vehicle speed sensor, thus leading to difficulties.
In addition, because it is possible to diagnose a fault of the input shaft speed sensor only when the shift lever is positioned in the forward running range in the prior art, the fault diagnosis method cannot be performed in an N range or a P range.
In case the ECU determines the fault of the input shaft speed sensor on the basis of the engine speed signal, the ECU detects whether the input shaft speed sensor is malfunctioning only when the engine speed is higher than the stall RPM because there is a possibility of a misjudgment in the engine stall state. Therefore, in a specific engine RPM range the fault diagnosis becomes impossible.
The present invention provides a fault diagnosis method in which the fault of the input shaft speed sensor of a vehicle without a vehicle speed sensor can be diagnosed even when the gearshift lever is not positioned in the forward running range or the engine speed is low. According to a preferred embodiment, the present invention comprises (a) determining whether a shift lever is positioned in a forward running range based on a signal of an inhibitor switch, (b) determining whether the input shaft speed sensor works properly by reading a first-state flag if the shift lever is determined to be not positioned in the forward running range, (c) determining whether the shift lever is positioned in a P range or an N range based on the signal of the inhibitor switch if the input shaft speed sensor works properly, (d) determining whether an automatic transmission fluid temperature is higher than a predetermined critical temperature if the shift lever is positioned in the P range or the N range, and (e) separately determining whether the input shaft speed sensor is faulty according to whether a turbine speed is higher than a predetermined first critical speed, if the temperature of the automatic transmission fluid is higher than the predetermined critical temperature.